1. Field of the Invention
The present invention relates to a semiconductor device and a driving method of the semiconductor device. In this specification, a semiconductor device refers to a semiconductor element itself or a device including a semiconductor element. As such a semiconductor element, a transistor is given, for example. Accordingly, the semiconductor device includes liquid crystal display devices, memory devices, and the like.
2. Description of the Related Art
As one kind of volatile memory, a static random access memory (SRAM) is widely known. For example, Patent Document 1 discloses a semiconductor integrated circuit in which data in an SRAM is saved in an external context holding portion, leading to lower power consumption and shorter starting time.
Meanwhile, in recent years, a metal oxide having semiconductor characteristics (hereinafter referred to as an oxide semiconductor) has attracted attention. Oxide semiconductors can be applied to transistors (Patent Documents 2 and 3).